Flat Roof Leaking in Heavy Rain?

1. The outlet: leaves and moss block it, water rises, and it finds the first weak lap or the top of a trim.
2. The wall: where the roof meets the house, the upstand or the flashing is too low or has pulled away. Light rain runs off; heavy rain splashes and rises above it.
3. Ponding: a dip holds water, and in heavy rain the pond spreads to a seam or a crack. See falls and ponding.
4. Wind-driven rain at edge trims, rooflight kerbs and the tops of upstands.
5. The gutter at the low edge overflowing back under the trim.

On a cold roof, with insulation between the joists and a void above it, warm moist air from the room can condense under the deck in cold weather and drip. That shows up on cold mornings, not only after heavy rain. A dry spell that still brings drips points to condensation; drips that follow the rain point to a leak. A warm roof moves the insulation above the deck and deals with it.

What you can do today

• From a window, look at the outlet after rain: is water sitting round it?
• Clear leaves from the outlet and the gutter, only if you can do it safely from a ladder.
• Put a bucket under the drip and move valuables. Note when it drips: during the rain, after it, or on cold dry mornings.
• Don't walk on a roof that feels soft. The deck may be rotten.